Acoustic signals in the environment are normally coupled to the inner ear through the action of the middle ear's tympanic-membrane - ossicular-chain linkage; however, the existence of other coupling mechanisms is regularly acknowledged. A clear demonstration of non-ossicular coupling of sound to the inner ear is the residual hearing observed after disruption of the ossicular chain. Otologists are generally taught that sound acting directly on the cochlear windows is an important factor in these cases. Nonetheless, formal models of the middle ear have ignored this mechanism, and physiologists usually explain residual hearing after ossicular disruption as "bone conduction", a rubric which encompasses a whole range of sound-coupling mechanisms. Generally the interactions of ossicular coupling, bone conduction and other forms of non-ossicular mechanisms are poorly understood. We have chosen to focus on air-borne acoustic stimulation of the cochlear windows ("acoustic coupling") because the physical mechanism is well defined and its importance can be tested. A model of the middle ear that includes acoustic coupling has been developed, and preliminary analysis of the model for three conditions involving ossicular interruptions indicates that acoustic coupling may be dominant under these conditions. The model involves assumptions that we will test and acousto-mechanical quantities that we will describe in order to better understand acoustic stimulation of the cochlear windows. We also will test the hypothesis that "acoustic coupling" is the dominant sound-conducting mechanism in human ears when ossicular coupling has been drastically disrupted. In addition we will determine whether the model predicts the interaction of ossicular-and acoustic-coupling in pathological situations in which both mechanisms may be important (e.g. perforations of the tympanic membrane). Each of the measurements will provide basic knowledge of middle-ear mechanical performance. To the extent that the hypothesis is supported we will have established a new way to think quantitatively about how different middle-ear coupling mechanisms combine to influence hearing in normal, pathological and surgically reconstructed ears.